Method of forming planarized coatings on contact hole patterns of various duty ratios

ABSTRACT

A method of forming a planarized photoresist coating on a substrate having holes with different duty ratios is described. A first photoresist preferably comprised of a Novolac resin and a diazonaphthoquinone photoactive compound is coated on a substrate and baked at or slightly above its Tg so that it reflows and fills the holes. The photoresist is exposed without a mask at a dose that allows the developer to thin the photoresist to a recessed depth within the holes. After the photoresist is hardened with a 250° C. bake, a second photoresist is coated on the substrate to form a planarized film with a thickness variation of less than 50 Angstroms between low and high duty ratio hole regions. One application is where the second photoresist is used to form a trench pattern in a via first dual damascene method. Secondly, the method is useful in fabricating MIM capacitors.

FIELD OF THE INVENTION

The invention relates to the field of fabricating semiconductor devicesand other electronic devices and in particular to a planarization methodused for the formation of semiconductor devices.

BACKGROUND OF THE INVENTION

The manufacture of integrated circuits in a semiconductor deviceinvolves the sequential deposition of layers in which patterns areformed. A pattern is first formed by a lithography process in aphotoresist layer and is subsequently transferred into one or morelayers in a substrate by an etching method. Alternately, the photoresistpattern can serve as a mask for an ion implant step. In either case, animportant requirement of the photoresist layer is forming a planarsurface in order to afford a large process latitude for the patterningstep.

Often the substrate upon which the photoresist is spin coated is notplanar because it may be comprised of a pattern containing features suchas lines that protrude above the surface of the substrate. Othersubstrates may have a largely level surface except for contact holes ortrenches that are etched below the surface. The topography of non-planarsubstrates can involve thickness variations as large as 1 micron ormore. However, even substrate thickness differences of only 0.1 to 0.2microns can be significant when considering that photoresist filmthickness is becoming thinner as feature size decreases. For advancedtechnology nodes where the critical dimension of a line width or spacewidth is less than 200 nm, most photoresist layers are in the range ofabout 2000 to 8000 Angstroms (0.2 to 0.8 micron) thick. A photoresistcomposition normally includes an organic solvent, a photosensitivecompound, and a polymer that has a low molecular weight which flowseasily and tends to planarize readily on relatively smooth surfaces.However, when the topography includes steps with a height that is morethan about 10 to 20% of the photoresist film thickness, thenplanarization of the photoresist film is difficult.

The photoresist film is exposed with radiation from an exposure sourcesuch as an excimer laser or a broadband Hg/Xe lamp that passes through amask containing the device pattern to be reproduced on the substrate.The mask has a patterned opaque coating such as chrome on a transparentsubstrate like quartz. A good lithography process is defined as one thathas a manufacturable process window in which there is a wide doselatitude and focus latitude for printing the pattern in the photoresistfilm. Generally, a depth of focus (DOF) of about 0.4 to 1 micron and adose latitude of at least 10 to 20% is desirable for maintaining aprinted feature size within ±10% of a targeted value. One can appreciatethat if the photoresist layer has a thickness variation of 0.1 micron ormore, then a significant portion of the DOF budget has been consumed bymaterial aspects and not by the exposure process itself. Typically, thespin coating process is optimized so that the photoresist thicknessvariation is minimized to a value of less than 10 Angstroms across thewafer. This is accomplished on planar substrates by varying the spinspeed during the coating process and by wetting the substrate with asolvent prior to applying the photoresist solution.

The relationship of photoresist thickness to the radiation dose orenergy required to print a pattern in the film is provided in FIG. 1.The plot of thickness vs. dose forms a sinusoidal curve 5 that hasminima 1, 3 and maxima 2, 4. This “swing” curve has an amplitude Abetween a minimum and a maximum energy and a periodicity B defined asthe distance (thickness) between two adjacent minimum points or twoadjacent maximum points on the curve 5. The magnitude of periodicity Bis related to the wavelength of the exposing radiation. The amplitude Ais calculated by dividing the difference between the energy for maximumpoint 2 (E₂) and the energy for minimum point 1 (E₁) by the average ofE₁ and E₂ which is (E₂−E₁)/(E₁+E₂/2) and this value can be as large as0.3 which is a swing of 30% in dose.

The swing effect is caused because radiation that passes through thephotoresist is partially reflected off the underlying layer and caneither constructively or destructively interfere with radiation making afirst pass through the photoresist. The extent of constructive ordestructive interference depends upon the thickness of the film and thewavelength of the radiation. The swing amplitude has a detrimentaleffect on the patterning process, especially if it is more than a few %of the average dose. Consider the condition in FIG. 1 where a swingamplitude of 30% is realized as determined previously for E₁ and E₂ andthe patterned feature is a contact hole. If a dose E₁ is used to form acontact hole in a photoresist that has a region with a thickness T₁ anda region with a thickness T₂, then the size of the hole with thicknessT₁ will be much larger than the hole size with thickness T₂ since thelatter requires a much higher energy to form a hole to a predeterminedsize. The size difference in space width of the hole is likely to bemuch greater than the ±10% specification described earlier for amanufacturing process.

In some situations, an anti-reflective coating (ARC) is applied to thesubstrate prior to the photoresist coating in order to controlreflectivity during the photoresist exposure step and enable a largerprocess latitude by reducing the swing effect. The ARC which can be anorganic or inorganic material is normally much thinner than thephotoresist and is most effective on relatively flat substrates. Whilesome organic ARCs have been developed for spin coating over featuressuch as contact holes, there are none available that can completelyplanarize a surface with topography variations of about 0.1 microns orlarger.

Planarization methods have been proposed for different applications inprior art. In U.S. Pat. No. 5,077,234, a process is provided for fillingSTI trenches of varying widths. The method requires three photoresistlayers. A first photoresist is patterned to fill only large trenches ofgreater than 30 microns in width. This photoresist plug is then hardenedby a combination of heating to 200° C. and UV exposure. A secondphotoresist is coated and baked to >150° C. and then etched back untilthe layer is removed over active regions. Then a third photoresist layeris coated and etched back to form a planar layer.

In U.S. Pat. No. 6,008,105, a process is described for planarizing adepression formed in an insulating layer that is deposited overinterconnect lines. A mask pattern is used to selectively leavephotoresist that fills the depression. The photoresist is baked at 150°C. to remove solvent and then cured by UV radiation. A secondphotoresist is coated on the substrate and etched back to form a planarsurface. This technique requires a new mask to be built for each patternof interconnect lines and can be expensive since several metal layersare present in a device.

U.S. Pat. No. 5,618,751 describes a method of forming a trench capacitorthat requires a photoresist to be coated over a trench that is <0.5microns wide. Since the opening is small, the photoresist does not fillthe trench and must be heated above its softening point so that it flowsinto the trench. The photoresist is preferably exposed with an electronbeam source to avoid diffraction effects and formation of standing waveson the sidewalls of the trench. The photoresist is developed to form arecessed layer within the trench that serves as an etch stop for etchingan adjacent diffusion source layer to a prescribed depth. A point ismade that the electron beam exposure yields a more planar photoresistsurface within the trench than photolithography with Deep UV (248 nm),i-line (365 nm) or mid UV (435 nm) radiation sources. However, thispatent does not mention a solution for forming a planar photoresist overa substrate that has both isolated and dense trench patterns.

Other background art found in U.S. Pat. No. 6,218,196 deals with aproblem of etching a pattern that contains both dense and isolatedlines. The space between dense lines etches slower than the region alongisolated lines and creates a reactive ion etch (RIE) lag. An apparatusand etching method are provided that includes a deposition gas such asCHF₃ that forms a protective layer on the photoresist sidewalls toprevent notching and an etching gas mixture of Cl₂ and BCl₃. Thereactive products from CHF₃ and Cl⁻ excessively react at isolated linesto produce a higher deposition rate that decreases the etch ratedifference between isolated and dense lines.

Besides the planarization requirement cited previously for dielectriclayers on interconnect lines, for filling STI trenches, and for formingtrench capacitors, another application shown in FIG. 2 that needs aplanar photoresist layer is a dual damascene process in which a trenchis patterned above a substrate 10 and etch stop layer 11 that includesboth isolated 13 a and dense via holes 13 b-13 e in dielectric layer 12.In this case, the photoresist 14 thickness over the dense via holeregion is thinner than over the isolated hole 13 a because aconsiderable amount of photoresist is used to fill the dense holes 13b-13 e. The difference in photoresist thickness is represented by thedistance D1 and can be over 2000 Angstroms. This is a large variationthat can reduce DOF for the patterning process and result in a largedifference in trench opening sizes that are formed above the holes. Onecurrent solution to the problem that is practiced by the inventors is toetch back the photoresist 14 and repeat the photoresist coating and etchback process several times in order to form a planar photoresist.However, this is costly in terms of slow throughput as well as materialand equipment usage. An improved method is needed that has fasterthroughput and has a minimal cost impact on the manufacturing scheme.

SUMMARY OF THE INVENTION

An objective of the present invention is to provide a photoresistplanarization process that is low cost and can be readily implemented ina manufacturing environment.

A further objective of the present invention is to provide a photoresistplanarization method that can be used in a dual damascene process wheretrenches are patterned above both isolated and dense via holes in thesame pattern.

A still further objective of the present invention is to provide aplanarization method that can be applied to fabricatingmetal-insulator-metal capacitors in which contact holes having regionsof low and high duty ratios exist in the same pattern.

According to one embodiment, these objectives are accomplished byproviding a substrate that has a patterned dielectric layer comprised ofboth isolated and dense via holes formed thereon. A first photoresistlayer is spin coated on the dielectric layer and baked at a high enoughtemperature so that the photoresist reflows into the holes and therebyforms an uneven thickness above the dielectric layer. The photoresist isblanket exposed without a mask and is developed to remove allphotoresist above the dielectric layer and form a recessed layer ofphotoresist within the holes. A high temperature bake of 250° C. isperformed to remove any remaining solvent in the photoresist and toharden the film. Preferably, the photoresist is comprised of a Novolacresin and a diazonaphthoquionone photoactive compound which form acrosslinked network that becomes impervious to organic materials orsolvents that are coated on it. Then a second photoresist is spin coatedon the dielectric layer having holes containing the recessed hardenedphotoresist to form a planar layer that can be controllably patternedwith trenches that are aligned above the contact holes. Conventionalprocessing is then followed to form metal interconnects.

In a second embodiment that relates to metal-insulator-metal (MIM)capacitor technology, a substrate is provided with a dielectric layerhaving contact hole regions with different duty ratios in which a bottomelectrode such as TiN has been deposited. A first photoresist layer isspin coated and baked at a high enough temperature so that thephotoresist reflows into the holes and thereby forms an uneven thicknessabove the metal layer. The photoresist is blanket exposed without a maskand developed to remove all photoresist above the metal layer except fora recessed layer of photoresist within the holes. A high temperaturebake of 250° C. is performed to remove any remaining solvent and toharden the film. Preferably, the photoresist includes a Novolac resinand a diazonaphthoquionone photoactive compound which form a crosslinkednetwork that becomes impervious to organic materials or solvents thatare coated on it. Then a second photoresist is spin coated on the metallayer and covers the holes containing the recessed hardened photoresistto form a planar layer. The photoresist is then etched back until itforms a recessed layer within the holes. A second etch is then performedto etch back the TiN layer until it is about coplanar with the recessedphotoresist layer. The remaining photoresist is then removed by a plasmaashing process and conventional processing is followed to complete theMIM capacitor.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a drawing illustrating a swing curve in which the energyrequired to pattern a photoresist layer varies with the photoresistthickness.

FIG. 2 is a cross-sectional view showing an uneven photoresist layerthat is formed after reflowing the resist into a pattern that containsisolated and dense via holes.

FIG. 3 is a cross-sectional view of the structure shown in FIG. 2 afterthe photoresist layer is blanket exposed and developed.

FIG. 4 is a cross-sectional view of the structure in FIG. 3 after ahardening step.

FIG. 5 is a cross-sectional view of the structure in FIG. 4 after asecond photoresist layer is spin coated to form a planar layer.

FIG. 6 is a cross-sectional view showing trench pattern formation in thesecond photoresist layer.

FIG. 7 is a cross-sectional view showing completion of a dual damascenestructure.

FIG. 8 is a cross-sectional view showing a metal-insulator-metal (MIM)device after a photoresist application on a substrate containing contactholes that are lined with a layer of metal.

FIG. 9 is a cross-sectional view after the photoresist layer in FIG. 8is blanket exposed and developed to form a recessed layer within theholes.

FIG. 10 is a cross-sectional view after the photoresist layer in FIG. 9is thermally hardened.

FIG. 11 is a cross-sectional view after a second photoresist is spincoated on the structure in FIG. 10 to form a planar layer.

FIG. 12 is a cross-sectional view after the photoresist in FIG. 11 isetched back to form a recessed layer within the holes.

FIG. 13 is a cross-sectional view after the metal liner is etched to alevel that is about coplanar with the photoresist layer in the contactholes.

FIG. 14 is a cross-sectional drawing of a MIM capacitor after aninsulating layer and the top electrode are deposited.

DETAILED DESCRIPTION OF THE INVENTION

Preferred embodiments of the present invention will be described withreference to the accompanying drawings. The description of thefabrication of a dual damascene structure and a MIM capacitor areprovided as an example and not as a limitation as to the scope of thepresent invention. For example, the embodiments refer to a method ofplanarizing a photoresist over contact hole patterns but the inventionis equally effective in planarizing patterns containing other featuressuch as trenches.

The first embodiment is illustrated in FIGS. 2-7. Referring to FIG. 2, asubstrate 10 is provided that typically contains a substructure (notshown) comprising conducting and insulating layers. The details of theselayers have been omitted in order to simplify the drawing and to focusattention on the key features of the present invention. An etch stoplayer 11 is formed on substrate 10 and is normally a material such assilicon nitride, silicon oxynitride, or silicon carbide that isdeposited by a technique such as chemical vapor deposition (CVD).

A dielectric layer 12 is deposited on etch stop 11 and is patterned byconventional means to generate via holes that include an isolated hole13 a and dense holes 13 b-13 e. Dielectric layer 12 is selected from agroup including SiO₂, carbon doped SiO₂, fluorosilicate glass,polysilsesquioxanes, polyarylethers, polyimides, and other low kdielectric materials, and has a thickness in the range of about 2000 to20000 Angstroms. Via holes 13 a-13 e can be formed with various dutyratios depending upon the device pattern. A duty ratio is defined as thespace occupied by the via holes divided by the total area of thepattern. The duty ratio approaches 0 for patterns containing onlyisolated holes and approaches 0.5 for a densely populated hole pattern.

The present invention is most effective for patterns having both lowduty ratio and high duty ratio regions. FIG. 2 is not necessarily drawnto scale and the distance between hole 13 a and its nearest neighboringhole 13 b may be more than 10 times the width of hole 13 a. It isunderstood that the pattern is more complex than represented by thedrawing and from a top-down view, the isolated and dense holes can formmany designs in an (x, y) coordinate grid that is bounded by the edge ofthe substrate.

A photoresist solution in an organic solvent is spin coated ondielectric layer 12 and partially dries while spinning but may notcompletely fill holes 13 a-13 e. When the substrate 10 is baked at about90° C. to 130° C. for up to 2 minutes, the partially dried photoresistlayer 14 reflows such that it completely fills via holes 13 a-13 e.Although photoresist layer 14 has a relatively thick film in the rangeof 5000 to 35000 Angstroms, the thickness in a region over holes 13 b-13e is less than the thickness in a region over isolated hole 13 a by adistance D1 shown in FIG. 2. A thickness variation D1 occurs because aconsiderably larger amount of photoresist 14 is needed to fill denseholes than an isolated hole. The magnitude of D1 can be as high as 2000Angstroms which is much larger than the desirable target of about 10Angstroms that is observed when forming a photoresist layer on a flatsurface. If photoresist layer 14 in FIG. 2 is patterned, then D1 willcause swing effects as described previously during an explanation of theswing curve in FIG. 1. In the case of trenches formed above via holes ina dual damascene scheme, the variation in the size of the resultingtrenches will be unacceptable if no planarization of the photoresistlayer is achieved before the patterning process.

Photoresist 14 is preferably a positive tone composition in which thesolubility of exposed regions changes. When the exposed pattern istreated with a developer that is usually an aqueous base solution, thenthe soluble exposed regions are washed away while any unexposed regionsremain insoluble and are not removed. Different types of positive tonecompositions are available and have been developed for a particularexposure wavelength such as Deep UV (248 nm), i-line (365 nm), or Mid UV(435 nm).

Preferably, the photoresist 14 is comprised of a Novolac resin and adiazonaphthoquinone (DNQ) photoactive compound which is useful forexposing wavelengths between about 300 and 500 nm. The Novolac resinthat comprises a majority of photoresist layer 14 generally has a glasstransition temperature (Tg) between about 90° C. and 130° C. thatenables the photoresist to reflow into holes 13 a-13 e following spincoating and baking at or slightly above the Tg. The DNQ compound andtraces of organic solvent in the resin matrix tend to lower the melttemperature of layer 14 below the glass transition temperature of theNovolac resin.

Referring to FIG. 3, the photoresist layer 14 in FIG. 2 is blanketexposed without a patterned mask and is then developed in an aqueousbase solution to remove all photoresist 14 above dielectric layer 12 andto form a recessed photoresist layer 14 a within the via holes 13 a-13e. Note that the level of photoresist 14 a in isolated hole 13 a ishigher than in holes 13 b-13 e since a thicker photoresist film 14covered 13 a during the exposure and therefore a smaller dose ofradiation reached via hole 13 a than penetrated holes 13 b-13 e. Thoseskilled in the art recognize that the radiation dose varies as afunction of depth into an absorbing film. Regions near the top of aphotosensitive layer receive a higher dose than regions near the bottomof the layer. In this example, the blanket exposure dose for photoresist14 is adjusted so that the region near the top of via holes 13 a-13 e inFIG. 2 receives a low dose while the lower part of holes 13 a-13 ereceives little or no dose. The dose that reaches the lower part ofholes 13 a-13 e is below a threshold value that is needed to convertenough of the DNQ compound to a base soluble material that makes layer14 soluble. Therefore, regions 14 a are insoluble in developer andremain in holes 13 a-13 e.

The blanket exposure dose for layer 14 can be delivered by a scanner orstepper exposure system or by a lower cost method involving a floodexposure tool that is available from suppliers like Fusion Systems. Abroadband Hg/Xe lamp exposes a substrate on a rotating stage duringflood exposure with a range of wavelengths from about 300 nm to about500 nm. A filter may be added to narrow the wavelength range thatexposes the substrate. Alignment capability is not necessary for thisstep.

Photoresist 14 a is then thermally hardened by heating the substrate 10on a hot plate at about 250° C. for about 2 minutes. A transformed layer14 b results as shown in FIG. 4 which consists of a crosslinked matrixcomprised Novolac polymer and a thermal product of DNQ. The thermaltreatment has removed any traces of organic solvent and water from layer14 b. As a result, layer 14 b is impervious to organic solvent andphotoresist that is coated on it.

Layer 14 b is likely to have a lower thickness in holes 13 a-13 e thanlayer 14 a since the hardening step compacts the matrix and removes anysolvents or water. The depth to which hardened layer 14 b is recessed inholes 13 a-13 e can vary. For a minimum depth, the hardened layer 14 bcan be coplanar with the top of hole 13 a. A maximum depth in holes 13b-13 e as indicated by H in FIG. 4 is limited by the ability to form aplanar layer on dielectric layer 12 in a subsequent photoresist coatingstep. Clearly, if H is too large, then an unacceptable thicknessvariation will also be realized in a second photoresist coating.Generally, a value of H which is from 0% to 30% of the depth of holes 13a-13 e is necessary to form a planar layer in the next step of thismethod.

A second photoresist is then spin coated on dielectric layer 12 andbaked in a range from about 90° C. to 150° C. to form a photoresistlayer 15 having a thickness between about 5000 and 35000 Angstroms asshown in FIG. 5. Typically, a solvent like propylene glycolmonomethylether acetate (PGMEA) or ethyl lactate is applied as the firststep in the photoresist coating process to enable a more uniform layer15 to be produced. This solvent does not interact with layer 14 bbecause of the previous hardening step. Layer 15 also fills holes 13a-13 e. A key feature is that layer 15 is essentially planarized becauseof the process outlined in FIGS. 2-4. For example, starting with a D1 of2060 Angstroms in a 3.4 micron thick photoresist layer 14, the methoddescribed in the first embodiment is followed to provide a photoresistlayer 15 with only a 50 Angstrom thickness variation.

Photoresist layer 15 is preferably a positive tone photoresist and ispatternwise exposed through a mask and developed to form trench openings16 a, 16 b, 16 c shown in FIG. 6. The exposure wavelength is selectedbased on the width of trench openings 16 a-16 c. If the width is largerthan about 300 nm, then an i-line exposure is generally preferred. For awidth in the range of about 130 nm to about 300 nm, then a Deep UVexposure is typically employed. For sub-130 nm openings, a sub-200 nmexposure wavelength such as a 193 nm or 157 nm wavelength from anexcimer laser source is preferred. Each of these wavelengths requires aphotoresist composition that has been tuned for that wavelength. Itshould also be noted that a thinner photoresist 15 thickness is neededas the width of trench openings decreases in order to maintain anadequate process window for the patterning step.

The unexposed portions of photoresist layer 15 serve as an etch maskduring a plasma etch to transfer the trench pattern partially throughdielectric layer 12. The trenches 16 a-16 c are aligned above via holes13 a-13 e. The alignment is not limited to the example shown in FIG. 6where one trench connects two via holes. In some designs, one trench isformed above each via hole and the present invention is equallyeffective in either case. The details of the etch process are not givenhere since they are well known to those skilled in the art.

Photoresist layers 15 and 14 b are then removed by a plasma ash processthat normally involves oxygen. An additional cleaning step may benecessary to remove any traces of photoresist residue before proceedingto fill the trenches and via holes with a conductive material. A barriermetal liner 17 as shown in FIG. 7 is deposited by a CVD technique on thesidewalls and bottom of trenches 16 a-16 c and of via holes 13 a-13 e.The liner 17 is typically comprised of a material such as Ti, Ta, W,TiN, TaN, TiW, or WN. Then a metal 18 is deposited by an electroplating,sputtering, or evaporating process to a level that is above the top ofdielectric layer 12. Metal 18 is preferably copper or a copper alloy,aluminum or an aluminum alloy, or tungsten or a tungsten alloy. Aplanarizing step such as chemical mechanical polishing is employed tolower the level of metal 18 so that it is coplanar with dielectric layer12 to complete the dual damascene structure pictured in FIG. 7.

The method shown in FIGS. 2-7 is an advantage over prior art in that aplanar photoresist 15 for the trench patterning step can be achievedwith a minimum amount of process time and materials. Existing tools andmaterials are utilized. The method is independent of trench and via holedesign and is effective for patterns containing via hole arrays withdifferent duty ratios. The coating, exposing, and developing ofphotoresist 14 can all be performed in one flow or job sequence and isespecially low cost if the blanket exposure is done with a relativelyinexpensive flood exposure tool rather than a more expensive scanner orstepper. An additional benefit is provided by the first photoresistlayer 14 that fills most of via holes 13 a-13 e in that the secondphotoresist layer 15 for patterning trenches does not require a highdose to reach the bottom of via holes 13 a-13 e. This allows a fasterthroughput during the exposure step that results in a lower costmanufacturing process. Furthermore, photoresist residues that are oftenformed in the bottom of via holes after the exposed photoresist isdeveloped are avoided by this method.

A second embodiment of the present invention is set forth in FIGS. 8-13with regard to fabrication of a metal-insulator-metal (MIM) capacitor.Referring to FIG. 8, a substrate 20 is provided that typically containsa substructure (not shown) comprising conducting and insulating layers.The details of these layers have been omitted in order to simplify thedrawing and to focus attention on the key features of the presentinvention. A dielectric layer 21 is deposited on substrate 20 and ispatterned by conventional means to generate contact holes that include alow duty ratio region containing hole 22 a and a high duty ratio regioncontaining dense holes 22 b-22 e. Dielectric layer 21 is selected from agroup including SiO₂, carbon doped SiO₂, fluorosilicate glass,polysilsesquioxanes, polyarylethers, polyimides, and other low kdielectric materials, and has thickness in the range of about 2000 to20000 Angstroms. Contact holes 22 a-22 e can be formed with various dutyratios depending upon the device pattern. A duty ratio is defined as thespace occupied by the contact holes divided by the total area of thepattern. The duty ratio approaches 0 for patterns containing onlyisolated holes and approaches 0.5 for a densely populated hole pattern.The present invention is most effective for patterns having contactholes with both low duty and high duty ratios. FIG. 8 is not necessarilydrawn to scale and the distance between hole 22 a and its nearestneighboring hole 22 b may be more than 10 times the width of hole 22 a.Moreover, the low duty ratio region represented by hole 22 a may consistof two or more holes grouped together. Only one hole 22 a is shown for alow duty ratio region to simplify the drawings. It is understood thatthe pattern is more complex than represented by the drawing and from atop-down view, the low duty ratio and high duty ratio regions can formmany designs in an (x, y) coordinate grid that is bounded by the edge ofthe substrate.

A conducting material that is preferably TiN or TIN/W is then depositedby a CVD technique and forms a conformal layer 23 on the surface ofoxide 21 and within the contact holes 22 a-22 e. The thickness of metallayer 23 which forms the bottom electrode of the MIM capacitor isbetween 100 and 500 Angstroms. Other suitable conducting materials suchas TaN and WN can also be used to form metal layer 23.

A photoresist solution in an organic solvent is spin coated on metallayer 23 and partially dries while spinning but may not completely fillholes 22 a-22 e. When the substrate 20 is baked at about 90° C. to 130°C. for up to 2 minutes, the partially dried photoresist layer 24 reflowssuch that it completely fills via holes 22 a-22 e. Although photoresistlayer 24 is a relatively thick film in the range of about 5000 to 35000Angstroms, the thickness in a region over holes 22 b-22 e is less thanthe thickness in a region over hole 22 a by a distance D2 shown in FIG.8. A thickness variation D2 occurs because a considerably larger amountof photoresist 24 is needed to fill holes in high duty ratio regionsthan in low duty ratio regions. The magnitude of D2 can be as high as2000 Angstroms which is much larger than the desirable target of about10 Angstroms that is observed when forming a photoresist layer on a flatsurface. A D2 of less than about 200 Angstroms is desired so that in asubsequent etch back step, the recessed depth of hardened photoresist 24a in hole 22 a will be approximately the same as the recessed depth oflayer 24 a within holes 22 b-22 e. This requirement will become clearerduring a description of FIGS. 11-12.

Photoresist 24 is preferably a positive tone composition in which theexposed regions become soluble in a developer. When the exposed patternis treated with a developer that is generally an aqueous base solution,then the soluble exposed regions are washed away while any unexposedregions remain insoluble and are not removed. Different types ofpositive tone compositions are available and have been developed for aparticular exposure wavelength such as Deep UV (248 nm), i-line (365nm), or Mid UV (435 nm).

Preferably, the photoresist 24 is comprised of a Novolac resin and adiazonaphthoquinone (DNQ) photoactive compound which is useful forexposing wavelengths between about 300 and 500 nm. The Novolac resinthat comprises a majority of photoresist layer 24 generally has a glasstransition temperature (Tg) between about 90° C. and 130° C. thatenables the photoresist to reflow into holes 22 a-22 e following spincoating and baking at or slightly above the Tg. The DNQ compound andtraces of organic solvent tend to lower the melt temperature of layer 24below the glass transition temperature of the Novolac resin.

Referring to FIG. 9, the photoresist layer 24 in FIG. 8 is blanketexposed without a patterned mask and is then developed in an aqueousbase solution to remove all photoresist 24 above metal layer 23 exceptfor a recessed photoresist layer 24 a within the contact holes 22 a-22e. Note that the level of photoresist 24 a in hole 22 a is higher thanin holes 22 b-22 e since a thicker photoresist film 24 covered 22 aduring the exposure and therefore a smaller dose of radiation reachedcontact hole 22 a than penetrated holes 22 b-22 e. Those skilled in theart recognize that the radiation dose varies as a function of depth intoan absorbing film. Regions near the top of a photosensitive layerreceive a higher dose than regions near the bottom of the layer. In thisexample, the blanket exposure dose for photoresist 24 is adjusted sothat the region near the top of via holes 22 a-22 e in FIG. 8 receives alow dose while the lower part of holes 22 a-22 e receives little or nodose. The dose that reaches the lower part of holes 22 a-22 e is below athreshold value that is needed to convert enough of the DNQ compound toa base soluble material that makes layer 24 soluble. Therefore, regions24 a are insoluble in developer and remain in holes 22 a-22 e.

The blanket exposure dose for layer 24 can be delivered by a scanner orstepper exposure system or by a lower cost method involving a floodexposure tool that is available from suppliers like Fusion Systems. Abroadband Hg/Xe lamp exposes a substrate on a rotating stage duringflood exposure with a range of wavelengths from about 300 nm to about500 nm. A filter may be added to narrow the wavelength range thatexposes the substrate. Alignment capability is not necessary for thisstep.

Photoresist 24 a is then thermally hardened by heating the substrate 20on a hot plate at about 250° C. for about 2 minutes. A transformed layer24 b results as shown in FIG. 4 which consists of a crosslinked matrixcomprised of Novolac polymer and a thermal product of DNQ. The thermaltreatment has removed any traces of organic solvent and water fromhardened layer 24 b. As a result, hardened layer 24 b is impervious toorganic solvent and photoresist that is coated on it.

Layer 24 b is likely to have a lower thickness in holes 22 a-22 e thanlayer 24 a since the hardening step compacts the layer and removes anysolvents or water. The depth to which hardened layer 24 b is recessed inholes 22 a-22 e can vary. For a minimum depth, the layer 24 b can becoplanar with the top of hole 22 a. A maximum depth in holes 22 b-22 eas indicated by H2 in FIG. 9 is limited by the ability to form a planarlayer on metal layer 23 in a subsequent photoresist coating step.Clearly, if H2 is too large, then an unacceptable thickness variationwill also be realized in a second photoresist coating. Generally, avalue of H2 which is from 0% to 30% of the depth of holes 22 a-22 e isnecessary to form a planar layer in the next step of this method.

A second photoresist is then spin coated on conducting layer 23 andbaked in a range from about 90° C. to 150° C. to form a photoresistlayer 25 having a thickness between about 5000 and 35000 Angstroms asshown in FIG. 10. Normally, a solvent like propylene glycolmonomethylether acetate (PGMEA) or ethyl lactate is applied as the firststep in the photoresist coating process to enable a more uniform layer25 to be produced. This solvent does not interact with hardened layer 24b because of the previous hardening step. Layer 25 also fills the holes22 a-22 e. A key feature is that layer 25 is essentially planarizedbecause of the process outlined in FIGS. 8-10. For example, startingwith a D2 of about 2000 Angstroms in a 3 micron thick photoresist layer24, the method described in the second embodiment is followed to providea photoresist layer 25 with about a 50 Angstrom thickness variation.

Photoresist layer 25 is not exposed in this process and can be anyphotoresist composition or even a polymer solution that is capable offorming uniform coatings of the required thickness mentioned previously.Photoresist 25 preferably has an etch rate similar to hardened layer 24.Photoresist 25 is subjected to an etch back step in which a plasma etchpreferably involving an oxygen gas is performed to remove all of layer25 above metal layer 23 and in contact holes 22 a-22 e. The etch alsoremoves some of hardened layer 24 b to give a recessed depth H3 in hole22 a and H4 in holes 22 b-22 e as shown in FIG. 11. Conditions for theetch are a O₂ flow rate of about 90 standard cubic centimeters perminute (sccm), an argon flow rate of about 20 sccm, a chamber pressureof 8 mTorr, a RF power of about 1200 Watts for a period of 120 seconds.

The desired magnitude of H3 and H4 are in a range of about 500 to 3000Angstroms. The benefit of the method of the second embodiment isapparent when comparing experimental results to a prior art method. Forexample, when photoresist 24 with a thickness variation D2 is etchedback by the process similar to the one described in the previousparagraph, then a recess depth in holes in low duty regions is about3690 Angstroms and the recessed depth in holes in high duty regions isabout 5600 Angstroms. The 1910 Angstrom difference is unacceptably largefor this device and will result in a MIM capacitor with a lowperformance. On the other hand, when the process illustrated by FIGS.8-11 is followed, then an H3 of 2280 Angstroms and an H4 of 2670Angstroms is provided in holes 22 a-22 e that have a total depth of14140 Angstroms. The difference of 390 Angstroms is acceptable for thisdevice and results in a MIM capacitor with a high performance.Therefore, the implementation of the second embodiment has significantlyimproved the fabrication method.

Optionally, prior art methods may involve several cycles of coating aphotoresist layer 24 and etching it back before D2 is reduced to anacceptable level. However, this technique is time consuming and costlyin terms of material and tool usage and increased substrate handling islikely to cause more defects. Therefore, the method described for FIGS.8-11 is more desirable in terms of lower cost and fewer defects.

The metal layer 23 is then plasma typically etched by a processcomprised of a 50 to 110 sccm flow rate of Cl₂, a chamber pressure of 5to 12 mTorr, and a RF power of 700 to 1500 Watts. This step lowers themetal layer 23 to a level that is about coplanar with the hardened layer24 b in holes 22 a-22 e. The importance of achieving a fairly even depthof H3 and H4 in FIG. 11 is that this condition enables the bottomelectrode 23 a to be recessed to a similar depth in holes 22 a-22 e asshown in FIG. 12.

The MIM capacitor is then completed by conventional steps of depositingan insulator layer 26 and a top electrode layer 27 as depicted in FIG.14 for an isolated capacitor fabricated from hole 22 a and then definingthe top plate pattern (not shown).

The method of the second embodiment can be readily implemented in amanufacturing environment since existing tools and materials areutilized. The method is independent of contact hole design and iseffective for patterns containing contact holes with different dutyratios. The coating, exposing, and developing of photoresist 24 can allbe performed in one flow or job sequence and is especially low cost ifthe blanket exposure is done with a relatively inexpensive floodexposure tool rather than a more expensive scanner or stepper.

While this invention has been particularly shown and described withreference to the preferred embodiments thereof, it will be understood bythose skilled in the art that various changes in form and details may bemade without departing from the spirit and scope of this invention.

We claim:
 1. A method for forming a planar photoresist layer on asubstrate with regions having holes of different duty ratios comprising:(a) providing a substrate with a layer comprised of a pattern containingholes in high duty ratio regions and in low duty ratio regions formedthereon, (b) coating a first photoresist layer on said substrate, (c)removing said photoresist from the surface of said substrate and forminga recessed layer of said photoresist in said holes, (d) hardening saidrecessed photoresist layer, and (e) coating a second photoresist layeron said substrate that forms a uniform thickness over holes in both highduty ratio regions and in low duty ratio regions.
 2. The method of claim1 wherein the low duty ratio regions are comprised of isolated holes andthe high duty ratio regions are comprised of dense holes wherein thespacing between holes is approaching the space width within the holes.3. The method of claim 1 wherein said first photoresist layer is apositive tone composition that includes a Novolac resin and adiazonaphthoquinone photoactive compound.
 4. The method of claim 3wherein said first photoresist layer is baked at a temperature slightlyabove the glass transition temperature of said first photoresist layer,preferably between about 90° C. and about 130° C.
 5. The method of claim4 wherein said photoresist is baked at a temperature such that saidfirst photoresist reflows and completely fills said holes.
 6. The methodof claim 3 wherein a recessed photoresist layer is formed in said holesby exposing said first photoresist with an appropriate exposure dose anddeveloping said photoresist.
 7. The method of claim 6 wherein said firstphotoresist is exposed without a mask using one or more wavelengths inthe range of about 300 nm to about 500 nm.
 8. The method of claim 6wherein the exposure is performed with a scanner or stepper tool thathas alignment capability or with a flood exposure tool that does nothave alignment capability.
 9. The method of claim 1 wherein the depth ofsaid recess in said holes is a distance that is from about 0% to 30% ofthe total depth of said hole.
 10. The method of claim 1 wherein athermal treatment is used to harden said first photoresist and comprisesa baking step at about 250° C. for about 2 minutes.
 11. The method ofclaim 1 wherein the second photoresist is a composition that issensitive to one or more wavelengths in the range from about 10 nm toabout 500 nm.
 12. A method for forming a planar photoresist layer on avia hole pattern in a dual damascene process comprising: (a) providing asubstrate with a stack of layers formed thereon, said stack comprisingan upper dielectric layer and a lower etch stop layer, said dielectriclayer includes a via hole pattern having holes in high and low dutyratio regions, (b) coating a first photoresist layer on said dielectriclayer, said photoresist is baked at a temperature such that thephotoresist reflows and fills said via holes, (c) removing saidphotoresist from the surface of said dielectric layer and forming arecessed layer of said first photoresist in said holes, (d) hardeningsaid recessed photoresist layer, and (e) coating a second photoresistlayer on said substrate that forms a uniform thickness over holes inboth high duty ratio regions and in low duty ratio regions.
 13. Themethod of claim 12 further comprised of completing a dual damascenestructure by exposing said second photoresist through a mask pattern toform trench openings that are aligned over said via holes, transferringsaid trench pattern into said dielectric layer with a plasma etch,removing first and second photoresists, forming a barrier metal liner insaid trenches and via holes, depositing a metal layer to fill saidtrenches and via holes, and planarizing said metal layer.
 14. The methodof claim 12 wherein said etch stop layer is preferably comprised ofsilicon nitride, silicon oxynitride, or silicon carbide.
 15. The methodof claim 12 wherein said dielectric layer is selected from a groupincluding SiO₂, carbon doped SiO₂, fluorosilicate glass,polysilsesquioxanes, polyarylethers, polyimides, and other low kdielectric materials.
 16. The method of claim 12 wherein said firstphotoresist layer is a positive tone composition that includes a Novolacresin and a diazonaphthoquinone photoactive compound.
 17. The method ofclaim 16 wherein said photoresist layer is baked at a temperatureslightly above the glass transition temperature of the layer, preferablybetween about 90° C. and about 130° C.
 18. The method of claim 12wherein said recessed photoresist layer is formed in said holes byexposing said first photoresist with an appropriate exposure dose anddeveloping said photoresist.
 19. The method of claim 18 wherein saidfirst photoresist is exposed without a mask using one or morewavelengths in the range of about 300 nm to about 500 nm.
 20. The methodof claim 18 wherein the exposure is performed with a scanner or steppertool that has alignment capability or with a flood exposure tool thatdoes not have alignment capability.
 21. The method of claim 12 whereinthe depth of said recess in said holes is a distance that is from 0% to30% of the total depth of the hole.
 22. The method of claim 12 wherein athermal treatment is used to harden said photoresist and comprises abaking step at about 250° C. for about 2 minutes.
 23. The method ofclaim 12 wherein the second photoresist is a composition that issensitive to one or more wavelengths in the range from about 10 nm toabout 500 nm.
 24. The method of claim 12 wherein the thickness variationin said second photoresist coating is less than about 50 Angstromsbetween high duty ratio regions and low duty ratio regions.
 25. A methodfor forming a planar photoresist layer on a contact hole pattern infabricating a metal-insulator-metal (MIM) capacitor comprising: (a)providing a substrate with a dielectric layer formed thereon, saiddielectric layer contains a contact hole pattern having holes in highand low duty ratio regions, (b) depositing a conformal metal layer onsaid dielectric layer and in said holes, (c) coating a first photoresistlayer on said metal layer, said photoresist is baked at a temperaturesuch that the photoresist reflows and fills said contact holes, (d)removing said photoresist from the surface of said metal layer andforming a recessed layer of said photoresist in said holes, (e)hardening said recessed photoresist layer, and (f) coating a secondphotoresist layer on said metal layer that forms a uniform thicknessover holes in both high duty ratio regions and in low duty ratioregions.
 26. The method of claim 25 further comprised of completing theMIM capacitor by etching back said second photoresist to form a recessedlayer within said holes, etching back said metal layer to be aboutcoplanar with said recessed first photoresist layer, removing said firstphotoresist layer, depositing an insulator layer, depositing a topelectrode layer, and defining a top plate pattern.
 27. The method ofclaim 25 wherein the dielectric layer is selected from a group includingSiO₂, carbon doped SiO₂, fluorosilicate glass, polysilsesquioxanes,polyarylethers, polyimides, and other low k dielectric materials. 28.The method of claim 25 wherein the metal layer is preferably TiN orTiN/W.
 29. The method of claim 25 wherein said first photoresist layeris a positive tone composition that includes a Novolac resin and adiazonaphthoquinone photoactive compound.
 30. The method of claim 25wherein said photoresist layer is baked at a temperature slightly abovethe glass transition temperature of the layer, preferably between about90° C. and about 130° C.
 31. The method of claim 25 wherein saidrecessed photoresist layer is formed in said holes by exposing saidfirst photoresist with an appropriate exposure dose and developing saidphotoresist.
 32. The method of claim 31 wherein said first photoresistis exposed without a mask using one or more wavelengths in the range ofabout 300 nm to about 500 nm.
 33. The method of claim 31 wherein theexposure is performed with a scanner or stepper tool that has alignmentcapability or with a flood exposure tool that does not have alignmentcapability.
 34. The method of claim 25 wherein the depth of said recessin said holes is a distance that is from about 0% to 30% of the totaldepth of the hole.
 35. The method of claim 25 wherein said recessedphotoresist is hardened by a thermal treatment comprises a baking stepat about 250° C. for about 2 minutes.
 36. The method of claim 25 whereinthe second photoresist is a composition that has an etch rate similar tothat of the hardened first photoresist layer.
 37. The method of claim 25wherein the thickness variation in said second photoresist coating isless than about 50 Angstroms between high duty ratio regions and lowduty ratio regions.
 38. The method of claim 25 wherein the secondphotoresist is a composition that is sensitive to one or morewavelengths in the range from about 10 nm to about 500 nm.